i am sorry to hear about your son. I know that you are devastated. All of us here have been where you are and quite honestly, feel the same way every time we hear of a new diagnosis. Know that you are in a good place for support and answers.
I will give a shot at your question and begin with we don't always know. I am not a carrier but my son has DMD. Why? Could be that I have eggs that are mutated (called genetic mosacisim) and he got a 'bad egg' (no pun intended!). Then there is the chance that you could be a carrier. If you are, there is a 50/50 chance that you would pass on a bad 'x' gene to your affected son and a good 'x' gene to your 8 y/o (you, as a woman have 2 'x' genes to pass on, either of the 2 can go to a boy as they have 1x (yours) and 1y(their fathers); so there is a 50/50 chance they could get either x). Or there is just the random mutation...it just happens. No one knows why.
I will tell you that this diagnosis comes with bad and yes, good. Please know that you are in a good place. There is hope on the horizon..soon we hope, but nothing is soon enough.
Let us know where you live, so that we can help you at the state level.
I am in Louisiana.
I hope I sort of answered your question!
We will add Tyler to our prayers.
Lori and Seph